The present invention relates to bubble level based tiltmeters, and has particular relation to apparatus for compensating for the effects of temperature variations in the fluid containing the sensed bubble.
In the tiltmeter, just as in a carpenter's level, the position of a bubble against a curved upper surface is used either as part of a feedback loop to level the apparatus, or as a measurement device to determine how far from level the apparatus is. Whereas in a carpenter's level, the position of the bubble is measured by comparing it against marks in the glass container, in the tiltmeter, platinum electrodes and an electrically conductive fluid are used (typical composition is a potassium iodide electrolyte in an ethyl alcohol solution). An electrode is placed at the bottom center of the tiltmeter with other electrodes placed on the top surface. When the tiltmeter is tilted all the way to the left, the bubble will completely cover an electrode located across the right top surface, and no current can get through to it from the bottom electrode. If the tiltmeter is tilted all the way to the right, a left top surface electrode will be similarly isolated. When the tiltmeter is level, the bubble will lie between the top electrodes, and both will be in electrical contact with the central electrode.
If the top electrodes are made sufficiently large, the bubble will cover more or less of each of these electrodes as the tiltmeter tilts, and this variation in bubble position will result in a corresponding variation in the conductance between the bottom electrode and each of the top electrodes. This conductance can be measured with very high precision, allowing correspondingly high precision in the measurement of how far from horizontal the tiltmeter has tilted.
However, bubble level based tiltmeters are temperature sensitive, showing different amounts of indicated tilt for the same actual tilt measured at different temperatures. There are two sources of this temperature sensitivity.
First, the conductivity of the fluid changes with temperature. This affects the output of the tiltmeter directly.
Second, and less directly, the viscosity of the fluid changes with temperature. This has less importance in a semi-static situation, where only the long term tilt is of interest (for example, on an earthquake fault or on the side of a volcano which is subject to eruption). However, when short term tilt is of interest (to obtain a measure of the magnitude of the ongoing event), the term being short enough to be considered a low frequency (or even high frequency) oscillation, the amplitude of the bubble's oscillation is viscosity dependent: a bubble in a high viscosity fluid, and in a container subjected to an oscillation, will have a greater excursion from vertical, and thus a lesser excursion from the center point of the tiltmeter than will a bubble in a low viscosity fluid.